1. Field of the Invention
The present invention relates to a heterojunction bipolar transistor (HBT) which has a double hetero structure and in which the emitter and collector regions are formed of a semiconductor material having a wider band gap than that of the material of the base region.
2. Description of the Related Art
A heterojunction bipolar transistor is good in both high-frequency and switching characteristics, and is therefore promising for use as a transistor which is adapted for use with microwaves or a transistor which is adapted to perform high-speed logical operations.
In general, a cutoff frequency f.sub.T is one of the indexes of the speed performance of a bipolar transistor. In the case of an npn transistor, the cutoff frequency f.sub.T is expressed as an inverse number of the transit time of electrons in an element. In order to obtain a high cutoff frequency f.sub.T, therefore, it is necessary to shorten the transit time of electrons. The transit time .tau. of electrons in an element is determined by the following factors: an emitter charging time .tau..sub.E ; a base transit time .tau..sub.B ; and the sum .tau..sub.C of the collector charging time and the collector transit time. Of these factors, the base transit time .tau..sub.B is not dependent on the current density and is substantially constant, while the emitter charging time .tau..sub.E decreases with the current density and takes a value far smaller than 1 psec when the transistor is operated with a high current density (i.e., when the transistor is operated with a current density which is in the region between the latter half of the order of 10.sup.4 A/cm.sup.2 and a value slightly exceeding 10.sup.5 A/cm.sup.2).
The sum .tau..sub.C of the collector charging time and the collector transit time will be considered. Like the emitter charging time, the collector charging time which is due to the collector junction capacitance decreases with an increase in the current density. However, the collector transit time cannot be fully decreased. Particularly in the case of a conventional semiconductor structure having a homo-junction defined by a p.sup.+ base and an n.sup.- collector, electrons travel through the high electric field of a collector depletion layer at a saturation velocity as small as 1.times.10.sup.7 cm/sec or less. Since the collector transit time is lengthened if the depletion layer widens, it is safe to say that the collector transit time increases in proportion to the width of the depletion layer. Therefore, the sum .tau..sub.C of the collector charging time and the collector transit time is the most influential factor for determining the cutoff frequency f.sub.T.
In order to reduce the sum .tau..sub.C of the collector charging time and the collector transit time, various proposals have been made up to the present. For example, R. Katoh et al. disclose an element construction effective in reducing the sum, in "A Self-Consistent Particle Simulation For (Al Ga)As/GaAs HBTs with Improved Base-Collector Structures", Technical Digest of the IEDM-International Electron Devices Meeting, Washington, D.C., Dec. 6-9, 1987, pp. 248 to 251. This reference shows an npn transistor wherein a p.sup.- -type layer is formed in that region of the n-type collector layer which is in contact with the base layer. That is, what the reference shows is a so-called HBT having a p.sup.- type collector.
FIG. 1 is a sectional view of the HBT disclosed in U.S. patent application Ser. No. 214,058. The HBT of this reference uses a material based on (Al Ga)As/GaAs and has a p.sup.- -type collector. The collector layer 32 of this element is made up of: an n.sup.+ -type GaAs layer 32a, an n.sup.- -type GaAs layer 32b, and a p.sup.- -type GaAs layer 32c, and forms a homo-junction in conjunction with a base layer 33. The impurity concentration in the P.sup.- -type GaAs layer 32c is lower than that of the n.sup.- -type GaAs layer 32b. The electric field in the collector region is weakened by the p.sup.- -n.sup.- junction. Since, therefore, the overshoot of the electron velocity occurs, the collector transit time of electrons can be such a small value as is not determined by the electron saturation velocity.
However, when the current density is in the latter half of the order of 10.sup.4 A/cm.sup.2, the electrons stored in the collector cannot be neglected though the electron velocity in the collector is high. Due to the space-charge effect caused by the stored electrons, the holes stored in the collector cannot be neglected, either. To explain this phenomenon, a Monte Carlo simulation was carried out. The calculation results of this simulation are shown in FIG. 2. As is shown in FIG. 2, the carrier profile of holes gradually extends into the region of the p.sup.- -type collector, and the depletion layer of the collector is narrowed, accordingly. Therefore, the collector junction capacitance increases with an increase in the current density. Since an increase in the collector junction capacitance results in a decrease in both the cutoff frequency f.sub..tau. and the maximum oscillation frequency f.sub.MAX, the switching characteristic of the transistor is greatly affected. Further, when the element is operated at a high current density, the heat generated by the element cannot be neglected, with the result that phonon scattering becomes more noticeable. Since, therefore, the electron velocity in the collector decreases, the collector junction capacitance is considered to increase at a higher rate in accordance with an increase in the current density. It should be also noted that the collector junction capacitance of a p.sup.- -type collector structure is higher than that of a n.sup.- -type collector structure.
In almost every region of the current density, a heterojunction bipolar transistor including a p.sup.- -type collector layer has a cutoff frequency higher than that of a heterojunction bipolar transistor including a n.sup.- -type collector layer. Therefore, the former heterojunction bipolar transistor is expected to operate at a higher speed than before, if both the cutoff frequency f.sub.T and the maximum oscillation frequency f.sub.MAX are prevented from decreasing in a high current density region.